PHP const vs define: What is the Difference?

In PHP, both the const keyword and the define() function are used to define constants, which are identifiers for simple values that cannot change during script execution. While they achieve similar results, they differ significantly in their execution timing, scope, flexibility, and performance. This article outlines the key differences between these two methods to help you understand when to use each in your PHP code.

Compile-Time vs. Run-Time Execution

The most fundamental difference between const and define() is how and when they are registered by the PHP engine:

• const is a language construct that is defined at compile-time. Because it is processed before the script runs, you cannot place const declarations inside conditional structures like if statements, loops, or try/catch blocks.
• define() is a function that is executed at run-time. Because it is evaluated when the script runs, you can conditionally define constants using if statements or inside loops.

// This is valid:
if (true) {
    define('STATUS', 'active');
}

// This will cause a syntax error:
if (true) {
    const STATUS = 'active'; 
}

Class Scope vs. Global Scope

Where you can declare and use these constants within your codebase also differs:

• const can be used inside a class to define class constants, which are accessed using the scope resolution operator (ClassName::CONSTANT_NAME). It can also be used in the global namespace.
• define() cannot be used inside a class definition. It always defines global constants (or constants within a specific namespace if explicitly prepended to the name), but it cannot define class-specific constants.

class User {
    // Valid:
    const ROLE = 'admin';
    
    // Invalid (will cause a syntax error):
    // define('ROLE', 'admin'); 
}

Dynamic Names

Sometimes you may need to generate the name of a constant dynamically.

• define() allows dynamic names, meaning you can use variables or string concatenation to determine the name of the constant at run-time.
• const requires a static identifier and does not accept variables or expressions for the constant name.

$prefix = 'APP_';
define($prefix . 'VERSION', '1.0.0'); // Valid and works

// This is not possible with const:
// const $prefix . 'VERSION' = '1.0.0'; 

Performance

Because const is parsed at compile-time rather than run-time, it is slightly faster than define(), which carries the overhead of a function call. While this performance difference is negligible in small applications, using const is generally preferred for micro-optimizations in large-scale frameworks and libraries where many constants are declared.